Cell cultures are used in fermentative processes to produce substances, in particular proteins. A distinction is made between processes in which the cell cultures are genetically unmodified and form their own metabolic products and processes in which the organisms are genetically modified in such a manner that they either produce a larger amount of their own substances such as proteins or produce foreign (heterologous) substances. The organisms producing the substances are supplied with a nutrient medium which guarantees the survival of the organisms and enables the production of the desired target compound. Numerous culture media are known for these purposes which enable an optimal cultivation of the specific host.
Protein biotherapeutics, like monoclonal antibodies, are considered as well-established drugs to treat serious illnesses and disease, such as cancer, multiple sclerosis and rheumatoid arthritis (see review from Leader et al. (Leader et al. 2008)). Critical molecular quality attributes of these highly active compounds have to be tightly monitored to ensure patient safety and functional efficacy. Unintended chemical modifications of the target protein can occur during whole production process starting from synthesis by microorganisms and cell cultures, during protein purification, formulation and storage. The most commonly observed chemical degradation pathways for protein pharmaceuticals are asparagine deamidation, aspartate isomerization (Wakankar and Borchardt, 2006; Diepold et al. 2012; Dengl et al. 2013), and oxidation (Li et al. 1995; Ji et al. 2009; Hensel et al. 2011). Recently, several studies reported an additional relevant yet enzyme-generated byproduct, so-called protein SV of recombinantly synthesized biotherapeutics (Khetan et al. 2010; Wen et al. 2009; Feeney et al. 2013). SVs are unintended amino acid replacements at genetically anticipated positions which originate either from intrinsic nucleotide mutations (Bridges, 2001) or alternative amino acid misincorporation during translation (Zeck et al. 2012). Translational misincorporation is thought to be caused by promiscuity of aminoacyl-tRNA synthetase (aaRS) for structurally related amino acids in the course of endogen substrate limitation (Feeney et al. 2013; Jakubowski, 2001). Misincorporation of free meta-tyrosine into cellular proteins as a potential cytotoxic mechanism for oxidized amino acids is reported in Gurer-Urhan et. al., (2006).